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  • Author Author: yuritikhonov
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Shedule & Homework Robot - Sound System's Hardware (part #2)

yuritikhonov

Good day comrade!

 

Today we'll talk about the audio hardware of my robot. Generally speaking, my sound system will consist of two parts:

  • melody generator;
  • speech generator.

 

The latter will be discussed in a future article, but today (and next week), our attention will be focused on the melody generator. So here's what I came up with:

image

The circuit operates as follows:

  • digital code of note comes from the MCU to the DAC;
  • DAC converts the note into an analog signal U = 0..5V;
  • analog signal U is fed to VCO, which converts it into frequency f = 0.5..10kHz;
  • frequency signal f from VCO is supplied with the power amplifier;
  • signal from the power amplifier enters the loudspeaker from which the sound is coming.

 

Pretty simple!

Circuitry of our "sound card" is very simple and clear:

image

Let's look at the purpose of each part:

  • IC2 - microcontroller generating digital codes;
  • R1-R10 and IC3A - improvised DAC;
  • IC1, R10-R13 and C1 - a simple VCO, built on classic 555 timer;
  • R14-R15 - frequency control (for fine tuning);
  • IC3B - power amplifier (U = const, I - enhanced);
  • SG1 - piezo speaker.

 

I’d like to explain one thing: DAC in this scheme is performed on the base  of  voltage divider, but you may have noticed that the scheme additionally uses Op-Amp IC3A. Why is it needed? Everything is very simple! In this case IC3A is used as a buffer stage to avoid the influence of signals from outside on the DAC.

 

Well, we’ve got the scheme, it's time to put it into practice! To begin with, I had to walk to the store and buy all the necessary electronic components:

image

Then came the hour of manufacturing PCB. The board is very simple, and I decided to draw it in not less simple program Sprint Layout:

image

Finally, I made the PCB. Traditionally, for this purpose, I used an iron and a laser printer:

image

And now it's time to test! The scheme worked at once, but an important drawback of piezo speaker was detected: these speakers have their own oscillation frequency (in my case 3.3kHz) and in case of applying to the speaker of a less frequency (e.g. 490Hz), the frequencies are summed:

image

In general, this does not prevent the system to work, but if I get a chance, I will get a normal speaker capable of reproducing clear sound.

 

Finally, I offer you a short video showing the work of my "sound card":

 

Next week, we will create music for our "sound card" using Matlab. See you soon!